I am increasingly of the opinion that you can either be eligible to vote, or collect pension, but not both at the same time. By the time you start collecting pension, you have effectively ousted yourself from economic production. And unlike a 20yo still in college and not contributing to the economy yet, you don't have 50 more years of your life to worry about. This effectively means it is safe for you to support any…
But at the end of the day, the point is moot: if non-pensioners can get their shit together enough to enact such a policy, they can surely organize themselves to oppose any poor voting by the pensioners.
The real problem you're facing is that young people: 1. Don't universally agree with you, and so support the policies you're blaming pensioners for, and, 2. Don't vote in large enough numbers to actually make their voices heard.
If you can fix that, you don't need to worry about disenfranchising a vulnerable portion of the population, since you can easily outvote them anyway.